Method and materials to convert a drilling mud into a solid gel based lost circulation material

ABSTRACT

A method for controlling loss circulation in a subterranean formation. The method includes circulating in a wellbore a nanosilica drilling fluid having a pH in a range of from about 8 to about 11 and a gel pH of less than 8, where the nanosilica drilling fluid includes an aqueous-based drilling mud and an alkaline nanosilica dispersion. The method also includes introducing into the nanosilica drilling fluid an amount of a chemical activator sufficient to produce a convertible drilling mud where the chemical activator is an acid and the pH of the convertible drilling mud is less than the gel pH. The method also includes allowing the convertible drilling mud to convert into the solid gel lost circulation material. A convertible drilling mud operable to convert into a solid gel lost circulation material is also provided.

BACKGROUND Field

Embodiments of the present disclosure generally relate to compositionsand methods for lost circulation control in a subterranean formation.

Technical Background

Fluids used in drilling a wellbore can be lost to the subterraneanformation while circulating in the wellbore. The drilling fluid canenter the subterranean formation via depleted zones, zones of relativelylow pressure, lost circulation zones having naturally occurringfractures, weak zones having fracture gradients exceeded by thehydrostatic pressure of the drilling fluid, and other openings betweenthe wellbore and the subterranean formation. The extent of fluid loss tothe subterranean formation can be greater than 100 barrels per hour(bbl/hr). As a result, the drilling service provided by such fluid ismore difficult or costly to achieve.

Current loss prevention methods require drilling to be temporarilyhalted so the drilling fluid can be replaced with a loss circulationpill. Periods of halted drilling directly result in non-productive timeand lost revenue.

SUMMARY

There is a continual need for a generation of an effective lostcirculation gel without requiring halting or termination of drillingoperations to replace the drilling fluid with a loss circulation pill.Disclosed are compositions and methods for lost control in asubterranean formation and specifically compositions and methods forproducing a lost circulation material in situ without halting drillingoperations for drilling fluid replacement.

According to one embodiment, a method for controlling loss circulationin a subterranean formation is provided. The method includes circulatingin a wellbore a nanosilica drilling fluid having a pH in a range of fromabout 8 to about 11 and a gel pH of less than 8. Further, the nanosilicadrilling fluid includes an aqueous-based drilling mud and an alkalinenanosilica dispersion. The method also includes introducing into thenanosilica drilling fluid an amount of a chemical activator sufficientto produce a convertible drilling mud where the chemical activator is anacid and the pH of the convertible drilling mud is less than the gel pH.The method also includes allowing the convertible drilling mud toconvert into the solid gel lost circulation material.

According to another embodiment, a convertible drilling mud operable toconvert into a solid gel lost circulation material is provided. Theconvertible drilling mud includes a nanosilica drilling fluid and achemical activator. The nanosilica drilling fluid includes an aqueousbased drilling mud and an alkaline nanosilica dispersion. The nanosilicadrilling fluid has a pH and a gel pH, and the pH of the nanosilicadrilling fluid is greater than the gel pH. The chemical activator has apH less than 7 and includes an acid that is operable to react with thenanosilica drilling fluid such that the solid gel lost circulationmaterial forms. Further, the volume ratio of the chemical activator tothe alkaline nanosilica dispersion is between 1:1000 and 1:10 beforeconversion of the convertible drilling mud into the solid gel lostcirculation material.

Additional features and advantages of the embodiments described will beset forth in the detailed description which follows, and in part will bereadily apparent to those skilled in the art from that description orrecognized by practicing the embodiments described, including thedetailed description which subsequently follows, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of the solid gel lost circulation material.

DEFINITIONS

In this disclosure, the term “lost circulation zone” refers to an areaencountered during drilling operations where the volume of drillingfluid returning to the surface is less than the volume of drilling fluidintroduced to the wellbore. The lost circulation zone can be due to anykind of opening between the wellbore and the subterranean formation.Lost circulation zones that can be addressed by convertible drilling muddescribed here can range from seepage loss to complete fluid loss.

In this disclosure, the term “gel pH” refers to the pH at which thesolid gel lost circulation material forms.

In this disclosure, the term “gel time” or “rate of gel formation”refers to the time it takes to form the solid gel lost circulationmaterial from the convertible drilling mud measured as the period fromwhen the gel pH is achieved until the solid gel lost circulationmaterial forms.

In this disclosure, the term “stable” refers to the state where thenanosilica particles are dispersed throughout the alkaline nanosilicadispersion and are not aggregated. Upon destabilization the nanosilicaparticles aggregate to form a gelled solid.

DETAILED DESCRIPTION

The present disclosure provides a method for controlling losscirculation in a subterranean formation. A well may be drilled with aconvertible drilling mud allowing for continued drilling uponencountering a lost circulation zone by converting the in-useconvertible drilling mud directly to a treatment fluid for curing thelost circulation. Specifically, the convertible drilling mud of thepresent disclosure includes an alkaline nanosilica dispersion, which maybe converted to a solid gel upon a pH reduction. The solid gel formed inthe lost circulation zone may then serve to prevent further fluid lossat the lost circulation zone by acting as a physical barrier.

The convertible drilling mud includes a nanosilica drilling fluid and achemical activator. The nanosilica drilling fluid includes an aqueousbased drilling mud and an alkaline nanosilica dispersion. The chemicalactivator is an organic or mineral acid such that exposure of thenanosilica drilling fluid to the chemical activator results in thenanosilica drilling fluid becoming destabilized and to form a gelledsolid. Advantageously, the convertible drilling mud can be used atelevated temperatures, such as those experienced in a wellbore and asubterranean formation.

The aqueous based drilling mud may be a water-containing fluid capableof use in a well drilling operation. The aqueous based drilling mud caninclude drilling fluid additives commonly used in drilling fluids. Thedrilling fluid additives can include viscosifiers, pH control agents,weighting agents, filtration control additives, and combinations of thesame.

The alkaline nanosilica dispersion represents a dispersion of nanosilicaparticles in a dispersion fluid in combination with an alkali, such asNaOH, to stabilize the dispersion. It will be appreciated that saltssuch as NaCl and KCl may be avoided in the alkaline nanosilicadispersion. The surface charge of the nanosilica particles may beneutralized by soluble salts that ionize and reduce the size of a doublelayer around the silica surface, which allows for aggregation. As such,maintenance of a minimized salt concentration is sought to ensureretention of the dispersion.

In one or more embodiments, the nanosilica particles in the alkalinenanosilica dispersion may have a particle size between 5 nanometers (nm)and 100 nm. In various further embodiments, the nanosilica particles inthe alkaline nanosilica dispersion may have a particle size of between 5nm and 80 nm, between 20 nm and 80 nm, between 30 nm and 60 nm, orbetween 40 nm and 50 nm. It will be appreciated that a smaller particlesize results in a relatively faster gelling when all other parametersare held constant.

The nanosilica particles may have a greater surface area than silicaparticles as a result of the smaller particle size. Without being boundto a particular theory, the greater surface area of the nanosilicaparticles can affect the rate of gelation and the nature of the gelsformed. More specifically, the smaller particle size of the nanosilicaparticles in the alkaline nanosilica dispersion promotes faster gellingthan silica particles.

The concentration of the nanosilica particles in the alkaline nanosilicadispersion may range between 5 percent by weight (wt %) and 50 wt %. Inone or more embodiments, the alkaline nanosilica dispersion may comprise10 wt % to 50 wt % nanosilica particles, 20 wt % to 50 wt % nanosilicaparticles, 30 wt % to 50 wt % nanosilica particles, or 40 wt % to 50 wt% nanosilica particles. In one or more specific embodiments, thealkaline nanosilica dispersion may comprise 43 wt % to 47 wt %nanosilica particles. The concentration of nanosilica in the alkalinenanosilica dispersion may impact the rate of gel formation with agreater concentration of nanosilica in the alkaline nanosilicadispersion correlating to a faster rate of gel formation. The amount ofthe alkaline nanosilica dispersion added to the aqueous based drillingmud may depend on the mud weight of the aqueous based drilling mud. Itwill be appreciated that an increase in the mud weight may necessitate acommensurate increase in nanosilica dispersion.

The chemical activator is an organic or mineral acid. Examples oforganic acids suitable to be the chemical activator include lactic acid,citric acid, maleaic acid, formic acid, and acetic acid. Examples ofmineral acids suitable to be the chemical activator include hydrochloricacid, sulfuric acid, nitric acid, and phosphoric acid. In one or morespecific embodiments, the chemical activator is citric acid. Thechemical activator may be in the form of powder, beads, aqueoussuspension, fibers, and combinations of the same.

The provided acid in the form of the chemical activator destabilizes thealkaline nanosilica dispersion to produce a solid gel lost circulationmaterial. The solid gel lost circulation material forms from networkstructures. The alkaline nanosilica dispersion may be stable at a pHbetween about 8 to about 12. The stability of the alkaline nanosilicadispersion is due to silica particle repulsion resulting from surfaceionization in the alkaline solution. The electrical repulsion betweenthe same charged particles stabilizes the dispersion. Disturbance of thecharge balance by reducing the pH to less than the gel pH due to theaddition of acid can cause the alkaline nanosilica dispersion todestabilize and aggregate, which results in the formation of the solidgel lost circulation material.

The alkaline nanosilica dispersion within the nanosilica drilling fluidmay be selected to obtain a gel pH compatible with the downholeconditions of the lost circulation zone and the gel time or rate of gelformation of the alkaline nanosilica dispersion. Specifically, ananosilica dispersion may be selected such that the convertible drillingmud may be conveyed to the lost circulation zone before the downholeconditions result in completion of the formation of the solid gel lostcirculation material from the convertible drilling mud. In variousembodiments, the gel pH may be less than 8, less than 7, less than 6, orless than 5.

The ratio between the alkaline nanosilica dispersion and the chemicalactivator in the convertible drilling mud may be between 0.1 percent byvolume (activator) of the total volume (nanosilica) (v/v %) to 10 v/v %.The weight ratio between the alkaline nanosilica dispersion and thechemical activator in the convertible drilling mud may alternatively bedefined as between 1 to 0.001 and 1 to 0.25. The amount of chemicalactivator added to the alkaline nanosilica dispersion may be adjustedbased on the change in pH required in the nanosilica drilling fluid. Agreater required pH change correlates to a greater v/v % or ratio ofchemical activator added to the alkaline nanosilica dispersion togenerate a greater volume of acid for pH adjustment.

The gel time determines the working period to position the convertibledrilling mud into the lost circulation zone after mixing of the chemicalactivator and the nanosilica drilling fluid. Specifically, introductionof the chemical activator into the nanosilica drilling fluid results ina virtually instantaneous adjustment of the pH of the resultingconvertible drilling mud to less than the gel pH leaving only the timefor actual solid gel formation to achieve the desired placement withinthe lost circulation zone. In one or more embodiments, the gel time mayrange between 5 minutes and 24 hours. In various further embodiments,the gel time may range between 30 minutes and 20 hours, 1 hour and 18hours, 2 hours and 15 hours, and 10 minutes to 10 hours. It will beappreciated that the gel time may be tuned to account for the wellboreconditions at the depth at which the lost circulation zone is located.

It will be appreciated that the downhole temperature may have an effecton the gel time of the alkaline nanosilica dispersion in the convertibledrilling fluid. Specifically, it is believed that as the surroundingtemperature increases the gel time decreases. Further, it is believedthat as the surrounding pressure increases the gel time decreases.Specifically, an electrical repulsion between the same charged particlesstabilizes the alkaline nanosilica dispersion and avoids gel formation.Disturbance of the charge balance, by changing the pH for example, cancause the colloidal particles to aggregate and initiate formation of agel. Increased pressure may increase the rate of aggregation of thesilica particles resulting in a decrease in the gel time. As such, thetemperature and pressure at the lost circulation zone must be accountedfor when preparing the convertible drilling fluid formulation and thevolume of chemical activator to utilize.

The solid gel lost circulation material is an irreversible solid gel.The solid gel lost circulation material does not degrade undertemperature, pressure or pH conditions once formed. Gel breakers do notbreak the solid gel lost circulation material either.

As indicated, the nanosilica drilling fluid may include drilling fluidadditives including viscosifiers, pH control agents, weighting agents,filtration control additives, and combinations of the same.

The nanosilica drilling fluid may optionally include one or morealkaline compounds for pH adjustment, which may include lime (calciumhydroxide or calcium oxide), soda ash (sodium carbonate), sodiumhydroxide (caustic soda), potassium hydroxide, or combinations thereof.It is noted that conjugate bases to acids with a pKa of more than about13 are considered strong bases. The pH may be maintained within a rangein order to minimize corrosion caused by the drilling fluid on steeltubulars, tanks, pumps, and other equipment contacting the drillingfluid as well as to maintain the pH greater than the gel pH prior tointroduction of the chemical activator. Additionally, the alkalinecompounds may react with gases, such as CO₂ or H₂S, encountered by thedrilling fluid during drilling operations to prevent the gases fromhydrolyzing one or more components of the drilling fluid. Some exampledrilling fluid compositions may optionally include from 0.1 pounds perbarrel (lb/bbl) to 10 lb/bbl of alkaline compounds. In some embodiments,the drilling fluid compositions includes from 0.1 lb/bbl to 10 lb/bbl ofcaustic soda, 0.1 lb/bbl to 1 lb/bbl of caustic soda, or 0.1 lb/bbl to0.5 lb/bbl of caustic soda.

In embodiments, the nanosilica drilling fluid may include a rheologymodifier, for example, a viscosifier, to impart non-Newtonian fluidrheology to the nanosilica drilling fluid to facilitate lifting andconveying rock cuttings to the surface of the wellbore and to suspendany weighting material. Examples of viscosifiers may include, but arenot limited to, xanthan gum polymer (XC polymer), bentonite,polyacrylamide, polyanionic cellulose, or combinations of theseviscosifiers. In some embodiments, the nanosilica drilling fluid mayinclude xanthan gum polymer, which is a polysaccharide secreted by thebacteria Xanthomonas Campestris (XC). An example nanosilica drillingfluid may include from 0.1 lb/bbl to 10 lb/bbl of XC polymer, 0.5 lb/bblto 5 lb/bbl of XC polymer, or 1 lb/bbl to 3 lb/bbl of XC polymer. Inanother example nanosilica drilling fluid, bentonite may be added to thedrilling fluid in an amount from 0.1 lb/bbl to 10 lb/bbl of bentonite, 1lb/bbl to 8 lb/bbl of bentonite, or 3 lb/bbl to 5 lb/bbl of bentonite.Other suitable viscosifiers may be used in the nanosilica drilling fluidwithout deviating from the scope of the present subject matter.

In one or more embodiments, filtration control additive may be added tothe nanosilica drilling fluid to reduce the amount of filtrate lost fromthe nanosilica drilling fluid into a subsurface formation. Examples offiltration control additives include organophilic (for example,amine-treated) lignite, bentonite, manufactured polymers, biopolymers,natural polymers, and thinners or deflocculants. In one or moreembodiments the filtration control additive may be a polyanioniccelluclose such as PAC-R™ which is commercially available fromHalliburton Energy Services, Inc. The filtration control additive mayalso be a starch. The nanosilica drilling fluid may include from 1lb/bbl to 10 lb/bbl of filtration control additive, from 2 lb/bbl to 8lb/bbl of filtration control additive, from 3 lb/bbl to 7 lb/bbl offiltration control additive, or from 4 lb/bbl to 5 lb/bbl of filtrationcontrol additive.

In further embodiments, additives in the nanosilica drilling fluid mayinclude a weighting material. The weighting material has a densitysuited for raising the drilling fluid composition density. In someembodiments, the weighting material may be a particulate solid having adensity sufficient to increase the density of the drilling fluidcomposition without adding excessive weighting material such that thenanosilica drilling fluid cannot be circulated through the wellbore. Theweighting material may have a density of from 2 grams per cubiccentimeter (g/cm³) to 6 g/cm³. Examples of weighting materials includebarite (minimum density of 4.20 g/cm³), hematite (minimum density of5.05 g/cm³), calcium carbonate (minimum density of 2.7-2.8 g/cm³),siderite (minimum density of 3.8 g/cm³), ilmenite (minimum density of4.6 g/cm³), or any combination of these weighting materials. Someexamples of nanosilica drilling fluid may include barite as the solid.It will be appreciated that the weight percent of the weighting materialin the nanosilica drilling fluid may be 0 to 99 wt. % based on the totalweight of the nanosilica drilling fluid to achieve an amount ofweighting material sufficient to acquire a particular desired density.

In at least one embodiment of the method for controlling losscirculation, the nanosilica drilling fluid may be utilized to drill awellbore in a subterranean formation. When a lost circulation zone isencountered, the chemical activator may be mixed with the nanosilicadrilling fluid to create the convertible drilling mud. It will beappreciated that the chemical activator can be introduced into thenanosilica drilling fluid while maintaining continuous flow of thenanosilica drilling fluid. The amount of chemical activator introducedinto the nanosilica drilling fluid may be adjusted to achieve a desiredgel time.

In one or more embodiments, the nanosilica drilling fluid and thechemical activator are combined at the surface of the wellbore andprovided downhole as a single convertible drilling mud. Specifically,the chemical activator may be introduced into the nanosilica drillingfluid to form the convertible drilling mud prior to introduction intothe wellbore. During passage from the surface to the lost circulationzone, acid in the chemical activator reduces the pH of the convertibledrilling mud and begins the process of converting the convertibledrilling mud to the solid gel lost circulation material. The acidreduces the pH of the convertible drilling mud as the convertibledrilling mud circulates to the lost circulation zone. In someembodiments, the pH of the convertible drilling mud is reduced to lessthan the gel pH prior to positioning in the lost circulation zoneresulting in initial formation of the solid gel lost circulationmaterial. In further embodiments, the solid gel lost circulationmaterial may begin to form subsequent to positioning in the lostcirculation zone as the pH of the convertible drilling mud is notreduced to less than the gel pH prior to positioning of the convertibledrilling mud within the lost circulation zone. The solid gel lostcirculation material fills the lost circulation zone or passagewaysleading to the lost circulation zone to cease lost circulation andprevent further drainage of the microsilica drilling fluid.

In one or more embodiments, the nanosilica drilling fluid and thechemical activator are provided downhole as separate process streams.Specifically, the chemical activator and the nanosilica drilling fluidmay be separately introduced into the wellbore, but combined to form theconvertible drilling mud prior to introduction into the lost circulationzone. Providing the nanosilica drilling fluid and the chemical downholeas separate process streams may be achieved by pumping the nanosilicadrilling fluid through the annulus of the drill string while thechemical activator may be pumped through the drill pipe. It will beappreciated that the nanosilica drilling fluid may instead be pumpedthrough the drill pipe and the chemical activator may instead be pumpedthrough the annulus. As such, conversion of the convertible drilling mudinto the solid gel lost circulation material is initiated prior tointroduction into the lost circulation zone. Similarly, to embodimentsin which the convertible drilling mud is formed prior to introductioninto the wellbore, the convertible drilling mud can circulate throughthe wellbore and channels within the subterranean formation to the lostcirculation zone positioned downstream of the mixing point of thenanosilica drilling fluid and chemical activator. As the convertibledrilling mud circulates through the wellbore and channels within thesubterranean formation toward the lost circulation zone, the acid in thechemical activator reduces the pH of the convertible drilling mud.Providing the constituents of the convertible drilling mud downholeseparately allows for positioning at a closer position to the lostcirculation zone before initiating the gelling process with the acid ofthe chemical activator. As with embodiments in which the convertibledrilling mud is formed prior to introduction into the wellbore, the pHof the convertible drilling mud may be reduced to less than the gel pHprior to positioning in the lost circulation zone or subsequent topositioning in the lost circulation zone. Upon formation, the solid gellost circulation material fills the lost circulation zone or passagewaysleading to the lost circulation zone to cease lost circulation andprevent further drainage of the microsilica drilling fluid.

The location of conversion to the solid gel lost circulation materialmay be determined based on calculations involving the volume of thenanosilica drilling fluid, the pump rate, and the distance between thelost circulation zone and the surface or mixing point of the nanosilicadrilling fluid and chemical activator. Further, it will be appreciatedthat when the convetible drilling mud reaches the lost circulation zone,circulation can be temporarily terminated so as to allow the convertibledrilling mud to gel in the lost circulation zone.

In one or more embodiments, when a lost circulation zone is encountered,a pill of the convertible drilling mud is produced by mixing thenanosilica drilling fluid and the chemical activator. The introductionof the nanosilica drilling fluid may be temporarily suspended withintroduction of the convertible drilling mud pill into the wellbore. Thepill of the convertible drilling mud may be allowed to migrate to thelost circulation zone. The pill of the convertible drilling mud may befollowed by a slug or pill of the nanosilica drilling fluid. The volumeof the pill of the convertible drilling mud may be based on the size ofthe lost circulation zone. It will be appreciated that the size of thelost circulation zone may be estimated based on the volume of lostdrilling fluid. Further, the amount of chemical activator may be basedon the desired gel time. As the pill of the convertible drilling mudmigrates to the lost circulation zone, the nanosilica dispersion in thenanosilica drilling fluid undergoes the conversion to a gelled solid.The solid gel lost circulation material fills the lost circulation zone.Advantageously, the chemical activator and the alkaline nanosilicadispersion do not precipitate from the aqueous based drilling mud, whichallows for the convertible drilling mud to be introduced to the wellboreas a single pill.

In one or more embodiments, the chemical activator may be combined withthe nanosilia drilling fluid within the lost circulation zone. Uponencountering a lost circulation zone with the nanosilica drilling fluid,the nanosilica drilling fluid will naturally flood the lost circulationzone. A slug or pill of the chemical activator may be provided to thelost circulation zone to form the convertible drilling mud within thelost circulation zone. It will be appreciated that when the slug or pillof the chemical activator reaches the lost circulation zone and mixeswith the nanosilica drilling fluid to form the convertible drilling mud,drilling and introduction of further drilling fluid can be temporarilyterminated so as to allow the convertible drilling mud to gel in thelost circulation zone.

EXAMPLE

Nanosilica Drilling Fluid Composition

A nanosilica drilling fluid was prepared as a 74.8 pounds per cubic foot(pcf) nanosilica drilling fluid representing Example 1. To prepare thenanosilica drilling fluid, four lb/bbl of bentonite was prehydrated in142.8 lb/bbl water for 16 hours and then mixed for 20 minutes (min).Additives were then added to the water/bentonite mixture according tothe order and formulation listed in Table 1 to form the nanosilicadrilling fluid.

TABLE 1 Formulation of nanosilica drilling fluid in Example 1. MixingAmount Component time (min) (lb/bbl) Water — 142.8 Bentonite 20 4 XCpolymer (biopolymer 5 2 viscosifier) Pre-gelatinized corn starch 5 4(filtration control additive) PAC-R ™ (filtration control 5 0.5additive) Caustic soda (pH control agent) 5 0.25 Alkaline nanosilicadispersion 5 265.81

For purposes of preparing Example 1, the alkaline nanosilica dispersionwas obtained commercially as IDISIL® SI 4545 from Evonik Industries(Essen, Germany); the properties are set forth in Table 2. The PAC-R™was obtained commercially from Halliburton (Houston, Tex.).

TABLE 2 Properties of alkaline nanosilica dispersion Property ValueParticle size - Titrated (nm) 45 Percent (%) SiO₂ 45 pH @ 25 degreesCelsius 9-11 (° C.) Density (g/cm³) 1.32 Viscosity @ 25° C. 30(centipoise (cP)) Visual Appearance white/off-white

For determination of the rheological and filtration properties ofExample 1, the 74.8 pcf nanosilica drilling fluid (Example 1) wasinitially hot rolled for 16 hours at 200° F. (93.33° C.) in an agingcell. After hot rolling for 16 hours, the nanosilica drilling fluid wasallowed to cool in the aging cell and then was transferred from theaging cell to a mud cup. The nanosilica drilling fluid was then mixedfor about 5 min in the mud cup after which rheological and filtrationproperties were measured. The rheological properties were measured usinga Fann 35 rheometer. The rheological measurements before hot rolling andafter hot rolling are shown in Table 3.

TABLE 3 Rheological and filtration properties of the 74.8 pcf nanocilicadrilling fluid of Example 1 Before Hot After Hot Rheology at 120° F.Rolling (BHR) Rolling (AHR) 600 revolutions 95 62 per minute (rpm) 300rpm 74 42 200 rpm 63 32 100 rpm 49 21 6 rpm 22 4 3 rpm 18 2 Plasticviscosity (cP) 21 20 Yield point (pounds per 53 22 square foot (lb/100ft²)) 10 sec. Gel 21 5 Strength (lb/100 ft²) 10 min. Gel 26 12 Strength(lb/100 ft²) pH 9.35 9.04 API fluid loss, 30 min (ml) 5

The nanosilica drilling fluid after hot rolling was demonstrated asstable. The nanosilica drilling fluid of Example 1 had a yield pointvalue of 22 lb/ft² and an API fluid loss, 30 min, of 5 ml after hotrolling. A drilling fluid having a yield point between 10 and 30 lb/100ft² is considered to be acceptable for industrial application. If theyield point is less than 10 the drilling fluid may not have sufficientcuttings carrying capacity, which may result in a stuck pipe.Additionally, for fluids having a density greater than 120 pcf in whichbarite is used as the weighting agent, the fluid may not have thecapacity to suspend barite resulting in barite sag. Further, a drillingfluid having an API fluid loss, 30 min, of less than 10 ml is consideredto be acceptable for industrial application. An elevated API fluid lossequates to greater drilling fluid being lost to the formation, requiringa greater amount of drilling fluid to drill the well.

Convertible Drilling Mud

A convertible drilling mud was prepared representing Example 2. Toprepare the convertible drilling mud, 5 grams (g) of chemical activatorwas added to 350 ml of the nanosilica drilling fluid prepared as Example1 in a mud cup. The specific chemical activator used was citric acid.The citric acid and nanosilica drilling fluid were mixed for 5 minutesusing a multimixer. The convertible drilling mud was then static aged ina vertical position at 250° F. (121.11° C.) and 1000 pounds per squareinch (psi) for 1 hour. After 1 hour of static aging, the convertibledrilling mud was converted to the solid gel lost circulation material asshown in FIG. 1.

It should be understood that the various aspects of the convertibledrilling mud and the method for producing a solid gel lost circulationmaterial using the same are described and such aspects may be utilizedin conjunction with various other aspects.

In a first aspect, the disclosure provides a method for controlling losscirculation in a subterranean formation. The method comprises a step ofcirculating in a wellbore a nanosilica drilling fluid having a pH in arange of from about 8 to about 11 and a gel pH of less than 8, thenanosilica drilling fluid comprising an aqueous-based drilling mud andan alkaline nanosilica dispersion. The method also comprises introducinginto the nanosilica drilling fluid an amount of a chemical activatorsufficient to produce a convertible drilling mud, where the chemicalactivator is an acid and where the pH of the convertible drilling mud isless than the gel pH. The method further comprises allowing theconvertible drilling mud to convert into the solid gel lost circulationmaterial.

In a second aspect, the disclosure provides the method of the firstaspect, in which the introduction of the chemical activator occurs uponthe wellbore fluidly contacting a lost circulation zone.

In a third aspect, the disclosure provides the method of the secondaspect, in which the chemical activator is introduced directly into thelost circulation zone such that the solid gel lost circulation materialforms in the lost circulation zone.

In a fourth aspect, the disclosure provides the method of the secondaspect, in which the chemical activator is introduced into the wellboresuch that the solid gel lost circulation material forms in the wellboreand the method further comprises the step of circulating the solid gellost circulation material into the lost circulation zone.

In a fifth aspect, the disclosure provides the method of the secondaspect, in which the chemical activator is introduced into the wellboresuch that the convertible drilling mud forms in the wellbore and themethod further comprises the step of circulating the convertibledrilling mud into the lost circulation zone such that the solid gel lostcirculation material forms in the lost circulation zone.

In a sixth aspect, the disclosure provides the method of any of thefirst through fifth aspects, in which the chemical activator comprisesan organic acid.

In a seventh aspect, the disclosure provides the method of any of thefirst through sixth aspects, in which the chemical activator is selectedfrom the group consisting of lactic acid, citric acid, maleic acid,formic acid, acetic acid, and combinations thereof.

In an eighth aspect, the disclosure provides the method of any of thefirst through fifth aspects, in which the chemical activator comprises amineral acid.

In a ninth aspect, the disclosure provides the method of any of thefirst through fifth or eighth aspects, in which the chemical activatoris selected from the group consisting of hydrochloric acid, sulfuricacid, nitric acid, phosphoric acid, and combinations thereof.

In a tenth aspect, the disclosure provides the method of any of thefirst through ninth aspects, in which the amount of the chemicalactivator introduced into the nanosilica drilling fluid is operable tomaintain a volume ratio of the chemical activator to the alkalinenanosilica dispersion of between 1:1000 and 1:10 before conversionoccurs.

In an eleventh aspect, the disclosure provides the method of any of thefirst through tenth aspects, in which the gel pH is less than 6.

In a twelfth aspect, the disclosure provides the method of the eleventhaspect, in which the chemical activator is operable to reduce the pH ofthe convertible drilling mud to less than 6.

In a thirteenth aspect, the disclosure provides the method of any of thefirst through twelfth aspects, in which a gel time is between 5 minutesand 24 hours.

In a fourteenth aspect, the disclosure provides a convertible drillingmud operable to convert into a solid gel lost circulation material. Theconvertible drilling mud comprises a nanosilica drilling fluid and achemical activator. The nanosilica drilling fluid comprises an aqueousbased drilling mud and an alkaline nanosilica dispersion, where thenanosilica drilling fluid has a pH and a gel pH, and where the pH of thenanosilica drilling fluid is greater than the gel pH. The chemicalactivator comprises an acid that is operable to react with thenanosilica drilling fluid such that the solid gel lost circulationmaterial forms, where the chemical activator has a pH less than 7.Further, the volume ratio of the chemical activator to the alkalinenanosilica dispersion is between 1:1000 and 1:10 before conversion ofthe convertible drilling mud into the solid gel lost circulationmaterial.

In a fifteenth aspect, the disclosure provides the convertible drillingmud of the fourteenth aspect, in which the chemical activator comprisesan organic acid.

In a sixteenth aspect, the disclosure provides the convertible drillingmud of the fifteenth aspect, in which the chemical activator is selectedfrom the group consisting of lactic acid, citric acid, maleic acid,formic acid, acetic acid, and combinations thereof.

In a seventeenth aspect, the disclosure provides the convertibledrilling mud of the fourteenth aspect, in which the chemical activatorcomprises a mineral acid.

In an eighteenth aspect, the disclosure provides the convertibledrilling mud of the seventeenth aspect, in which the chemical activatoris selected from the group consisting of hydrochloric acid, sulfuricacid, nitric acid, phosphoric acid, and combinations thereof.

In a nineteenth aspect, the disclosure provides the convertible drillingmud of any of the fourteenth through eighteenth aspects, in which thenanosilica drilling fluid has a pH between 8 and 11.

In a twentieth aspect, the disclosure provides the convertible drillingmud of any of the fourteenth through nineteenth aspects, in which thegel pH is greater than 3 but less than 8.

In a twenty-first aspect, the disclosure provides the convertibledrilling mud of any of the fourteenth through twentieth aspects, inwhich the chemical activator has a pH of less than 6.

It should be apparent to those skilled in the art that variousmodifications and variations can be made to the embodiments describedthroughout the present disclosure without departing from the spirit andscope of the claimed subject matter. Thus it is intended that thespecification cover the modifications and variations of the variousembodiments described throughout the present disclosure, provided suchmodification and variations come within the scope of the appended claimsand their equivalents.

The various elements described can be used in combination with all otherelements described here unless otherwise indicated.

The singular forms “a”, “an” and “the” include plural referents, unlessthe context clearly dictates otherwise.

“Optional” or “Optionally” means that the subsequently described eventor circumstances may or may not occur. The description includesinstances where the event or circumstance occurs and instances where itdoes not occur.

Ranges may be expressed here as from about one particular value to aboutanother particular value and are inclusive unless otherwise indicated.When such a range is expressed, it is to be understood that anotherembodiment is from the one particular value to the other particularvalue, along with all combinations within said range.

As used here and in the appended claims, the words “comprise,” “has,”and “include” and all grammatical variations thereof are each intendedto have an open, non-limiting meaning that does not exclude additionalelements or steps.

What is claimed is:
 1. A method for controlling loss circulation in asubterranean formation, the method comprising the steps of: circulatingin a wellbore a nanosilica drilling fluid having a pH in a range of fromabout 8 to about 11 and a gel pH of less than 8, the nanosilica drillingfluid comprising an aqueous-based drilling mud and an alkalinenanosilica dispersion; introducing into the nanosilica drilling fluid anamount of a chemical activator sufficient to produce a convertibledrilling mud, where the chemical activator is an acid and where the pHof the convertible drilling mud is less than the gel pH; and allowingthe convertible drilling mud to convert into the solid gel lostcirculation material.
 2. The method of claim 1, where the introductionof the chemical activator occurs upon the wellbore fluidly contacting alost circulation zone.
 3. The method of claim 2, where the chemicalactivator is introduced directly into the lost circulation zone suchthat the solid gel lost circulation material forms in the lostcirculation zone.
 4. The method of claim 2, where the chemical activatoris introduced into the wellbore such that the solid gel lost circulationmaterial forms in the wellbore; and where the method further comprisesthe step of circulating the solid gel lost circulation material into thelost circulation zone.
 5. The method of claim 2, where the chemicalactivator is introduced into the wellbore such that the convertibledrilling mud forms in the wellbore; and where the method furthercomprises the step of circulating the convertible drilling mud into thelost circulation zone such that the solid gel lost circulation materialforms in the lost circulation zone.
 6. The method of claim 1, where thechemical activator comprises an organic acid.
 7. The method of claim 6,where the chemical activator is selected from the group consisting oflactic acid, citric acid, maleic acid, formic acid, acetic acid, andcombinations thereof.
 8. The method of claim 1, where the chemicalactivator comprises a mineral acid.
 9. The method of claim 8, where thechemical activator is selected from the group consisting of hydrochloricacid, sulfuric acid, nitric acid, phosphoric acid, and combinationsthereof.
 10. The method of claim 1, where the amount of the chemicalactivator introduced into the nanosilica drilling fluid is operable tomaintain a volume ratio of the chemical activator to the alkalinenanosilica dispersion of between 1:1000 and 1:10 before conversionoccurs.
 11. The method of claim 1, where the gel pH is less than
 6. 12.The method of claim 11, where the chemical activator is operable toreduce the pH of the convertible drilling mud to less than
 6. 13. Themethod of claim 1, where a gel time is between 5 minutes and 24 hours.14. A convertible drilling mud operable to convert into a solid gel lostcirculation material, the convertible drilling mud comprising: ananosilica drilling fluid, the nanosilica drilling fluid comprising: anaqueous based drilling mud; and an alkaline nanosilica dispersion, wherethe nanosilica drilling fluid has a pH and a gel pH, and where the pH ofthe nanosilica drilling fluid is greater than the gel pH; and a chemicalactivator, the chemical activator comprising: an acid that is operableto react with the nanosilica drilling fluid such that the solid gel lostcirculation material forms, where the chemical activator has a pH lessthan 7; and where the volume ratio of the chemical activator to thealkaline nanosilica dispersion is between 1:1000 and 1:10 beforeconversion of the convertible drilling mud into the solid gel lostcirculation material.
 15. The convertible drilling mud of claim 14,where the chemical activator comprises an organic acid.
 16. Theconvertible drilling mud of claim 15, where the chemical activator isselected from the group consisting of lactic acid, citric acid, maleicacid, formic acid, acetic acid, and combinations thereof.
 17. Theconvertible drilling mud of claim 14, where the chemical activatorcomprises a mineral acid.
 18. The convertible drilling mud of claim 17,where the chemical activator is selected from the group consisting ofhydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, andcombinations thereof.
 19. The convertible drilling mud of claim 14,where the nanosilica drilling fluid has a pH between 8 and
 11. 20. Theconvertible drilling mud of claim 14, where the gel pH is greater than 3but less than
 8. 21. The convertible drilling mud of claim 14, where thechemical activator has a pH of less than 6.